A laser having a downstream amplitude modulator (intensity modulator) generates an optical spectrum with a spectral line at the carrier frequency and a bandwidth which is wider than the bandwidth of the information signal. Such an emission spectrum reduces the amount of information which can be transferred overall in a wavelength-division multiplex system. On account of the non-linear properties of the transmission fiber, a wider bandwidth results in greater distortion of the transfer signal. One method for reducing the bandwidth is duobinary coding. Such a method is described in IEEE Photonics Technology Letters, 1998, Vol. 10, Issue 8, pp. 1159-1161: “Prechirped Duobinary Modulation” by A. Djupsjöbacka (Ericsson Components, Sweden). However, this type of coding is very complicated.